Data centers and computing machine rooms need a large cooling system to cool servers, memory equipment, and network devices. A chilled water system is widely employed in all places that need long-term cooling the whole year, such as a large-scale data center, a switch room, etc. In low-temperature climate areas, the cooling systems in many of those places adopt a natural cooling technology. In a transition season or low-temperature season, the chilled water systems can use an outdoor cold source for free, without activating a chilled water main machine, thereby reducing a great deal of electricity usage for users. In most of data centers, the supplied and returned water temperatures of their chilled water systems are 10 and 15° C., respectively, and temperatures of air of its air conditioners are 13° C. Because of the relatively low supplied and returned water temperatures, the time in which a natural cooling technology may be utilized is relatively short.
A conventional chilled water natural cooling system includes a combination of an air-cooled chilled water main machine and a natural cooler, which is, generally, a closed cooling tower. As shown in FIG. 1, an air-cooled chilled water main machine 11 is connected in series with a natural cooler 10 and to a water supplying/returning end of a heat exchanger 12 of an air-conditioning terminal through a three-way valve 13. Meanwhile, the three-way valve 13 is further connected to a pipeline by which the air-cooled chilled water main machine 11 is connected in series with the natural cooler 10 through a valve 14.
In a mechanical cooling mode, when an outdoor air temperature exceeds a returned water temperature (20° C.), chilled water will absorb, not dissipate, heat if it still passes through the natural cooler 10. Thus, under the circumstance, the three-way valve 13 is adjusted to a bypass mode, in which the valve 14 is opened, and returned water of the heat exchanger 12 at the air conditioning terminal does not pass the natural cooler 10, but runs through the air-cooled chilled water main machine 11. The air-cooled chilled water main machine 11 is started to cool the 20° C. chilled water to 15° C. by the mechanical cooling process (including activating a compressor, dissipating heat by a condenser, etc.). The chilled water of 15° C. is sent back to the heat exchanger 12 at the air-conditioning terminal.
In a natural cooling mode, an outdoor air temperature must be less than a temperature of the supplied water. For example, the temperature of the supplied water is 15° C. and the outdoor temperature is 12° C. Chilled water of 15° C. is supplied to heat exchanger 12 and warmed up to 20° C. after absorbing heat load from the air-conditioning terminal. The three-way valve 13 is adjusted to enable the chilled water to pass through the natural cooler 10 so that the 20° C. chilled water is cooled to 15° C. through the outdoor natural cooler 10, and in turn passes through the air-cooled chilled water main machine 11, which does not need to be activated to cool. The chilled water is then sent back to the heat exchanger 12 of the air-conditioning terminal, thereby forming a chilled water loop.
In a partial natural-cooling mode, a chilled water circulation loop is identical to that of natural cooling, but an outdoor air temperature is not low enough to cool the chilled water to 15° C. after it passes through the natural cooler 10. The air-cooled chilled water main machine 11 needs to be activated to cool the chilled water to 15° C. The chilled water of 15° C. is then supplied to the heat exchanger 12 of the air-conditioning terminal.
At present, temperatures of the vast majority of data centers are set to be very low. For example, when natural cooling systems are provided in the cooling system of the data centers, generally, they are designed to have the supplied and returned water temperature at 15° C. and 20° C., respectively. In those systems, the natural cooling mode will be turned on when the outdoor temperature is about 10° C.; the partial natural cooling mode is turned on when the outdoor temperature is about 18° C.; and the mechanical cooling mode is activated when the outdoor temperature is more than 18° C.
Thus, the chilled water in the conventional chilled water loop always passes through the chilled water main machine 11 and selectively enters the natural cooler 10 under the control of the three-way valve 13. That is, the chilled water main machine 11 is the primary cooling source, while the natural cooler 10 is employed to assist main machine 11. As the technology for information devices such as IT servers advances, devices such as servers gradually do not require to be maintained at a lower temperature. For example, the air temperature is recommended to be at 18-27° C. in the TC9.9 specification recently published by the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). As the acceptable air temperature for the server is increased, the supplied and returned water temperatures of the chilled water in a cooling system may also be higher. The conventional cooling systems that primarily use the chilled water main machine 11 may not be sufficient to conserve energy.